Exploiting multiple exciton effects in organic solar cells

太阳是地球上最丰富的潜在可持续能源。使用有机导电聚合物将光转化为电能的太阳能电池？有机光伏(OPV)设备--为可再生电力生产提供了一条潜在的低成本途径。然而，为了实现与其他太阳能光伏技术的平起平坐，有机太阳能电池必须提高其电能转换效率。该项目将在有机聚合物太阳能电池装置中加入光活性材料，以促进称为三重态融合的量子力学过程。人们认为，三重态聚变可以提高功率输出的电压，从而导致更高的太阳能转换效率。这一概念从科学角度来看是创新的，因为它将专注于增加电压电位，而不是光电流，从而导致一条通过提高热力学效率来提高太阳能电池效率的途径。与该项目相关的教育活动包括参加普林斯顿大学材料学院，该学院向来自新泽西州特伦顿的未被充分代表的高中生教授动手材料科学，并通过在当地图书馆举行的纳米材料科学日，鼓励学生、教师和家长与太阳能科学家见面，并参与动手演示，以制造和测试简单的太阳能电池设备。这项研究的总体目标是开发一种新的、基于有机聚合物的激子太阳能光伏器件架构，通过三重态聚变过程，具有超越经典热力学功率转换效率极限的潜力。在三重态聚变中，需要两个由两个低能亚带隙光子形成的激子才能产生一个较高能量的光子。将敏化磷光材料集成到有机光伏(OPV)器件体系结构中，为实现三重态融合提供了手段。在磷光敏化的OPV器件中，吸收和单重态的形成主要发生在荧光主体给体上。在这个初始吸收事件之后，能量转移到低浓度下存在于施主层中的磷光客体。然后，磷光客体上的激子转移到宿主的三重态水平，从而允许荧光材料中的长寿命三重态的布居。这种三重态聚变的过程将被用来产生比载流子起源的激子能量更高的自由电荷载流子。本质上，这种器件结构代表了中间带太阳能电池的分子模拟，但具有与基于单线态裂变的太阳能电池相似的限制效率。三重态聚变装置与传统的中间带太阳电池的主要区别在于，在三重态聚变装置中，中间带是通过两个低能量的三重态激子聚变成一个较高能量的单重态激子来满足的。此外，单线态裂变装置希望增加光电流，而基于三重态聚变的装置则希望增加光电压，从而从根本上提高热力学效率。为了探索这一新现象，本研究有两个主要目的。第一个目标是展示磷光敏化OPV器件实现三重态融合的功能，第二个目标是通过全面的电学和光学表征来了解多个激子器件涉及的效率限制机制。最终，三重融合OPV设备的展示增加了另一个超越Shockley-Queisser限制的选择，并启发了高效率有机太阳能电池的工作。

The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use organic conducting polymers to convert light to electricity ? organic photovoltaic (OPV) devices - offer a potentially low-cost route for renewable electricity production. However, in order to achieve parity with other solar photovoltaic technologies, organic solar cells must increase their power conversion efficiency. This project will incorporate light-active materials into the organic polymer solar cell device to promote a quantum mechanical process called triplet fusion. It is believed that triplet fusion can increase the voltage of the power output, leading to higher solar energy conversion efficiency. This concept is innovative from a scientific point of view because it will focus on increasing the voltage potential, not the photocurrent, leading to a pathway to increase solar cell efficiency through an increase in thermodynamic efficiency. The educational activities associated with this project include participation in the Princeton University Materials Academy that teaches hands-on materials science to under-represented high school students from the Trenton, New Jersey, and multi-generational outreach through the Nano Materials Science Day at a local library that encourages students, teachers, and parents to meet with solar scientists and participate in hands-on demos to make and test simple solar cell devices. The overall goal of the research is to develop a new, organic polymer-based excitonic solar photovoltaic device architecture that has the potential exceed classical thermodynamic power conversion efficiency limits through the process of triplet fusion. In triplet fusion, two excitons formed from two low-energy sub band gap photons are required to produce one higher-energy photon. The integration of sensitized phosphorescent materials into the organic photovoltaic (OPV) device architecture offers the means to achieve triplet fusion. In a phosphor-sensitized OPV device, absorption and singlet formation occurs primarily on a fluorescent host donor. This initial absorption event is followed by energy transfer to a phosphorescent guest present in the donor layer at low concentration. Excitons on the phosphorescent guest then transfer to the triplet level of the host, permitting the population of the long-lived triplet state in the fluorescent material. This process of triplet fusion will be used to create free charge carriers with higher energy than the exciton from which the carriers originated. In essence, this device configuration represents the molecular analogue to an intermediate band solar cell, but possesses similar limiting efficiencies as singlet fission-based solar cells. The main distinction between the triplet fusion device and a conventional intermediate band solar cell is that, in the triplet fusion device, the intermediate band is satisfied via the fusion of two low energy triplet excitons into a higher energy singlet exciton. Furthermore, whereas the singlet fission device looks to increase photocurrent, the one based upon triplet fusion looks to increase the photovoltage, leading to fundamental enhancement of the thermodynamic efficiency. To explore this new phenomenon, the research has two major objectives. The first objective is to demonstrate the functionality of the phosphor-sensitized OPV device for achieving triplet fusion, and the second objective is to understand efficiency-limiting mechanisms involved with multiple exciton devices through comprehensive electrical and optical characterization. Ultimately, the demonstration of a triplet fusion OPV device adds another option to exceed Shockley-Queisser limits and inspire work toward high efficiency organic solar cells.